Distribution of 2,4,5,2',4',5'-hexachlorobiphenyl among lipoproteins during pregnancy and lactation in the rat.

The mechanism by which polychlorinated biphenyls are transferred from adipose tissue to the mammary gland during late pregnancy and lactation is unknown. Lipoproteins were investigated as a possible vehicle of transport. 2,4,5,2',4',5'-[14C] Hexachlorobiphenyl (6-CB) distribution among very low-, low- and high-density lipoproteins and the protein-rich bottom fraction was examined in virgin controls, in pregnant animals on days 9 and 18 of gestation, in mothers on day 10 of lactation and in suckling pups. Plasma obtained 1 hr after an i.v. injection of 6-CB was separated into the various lipoprotein fractions and their lipid composition analyzed. With advancing pregnancy, there was a shift in 6-CB distribution from the higher to lower density lipoproteins. A linear increase was observed in the amount of very low-density lipoprotein as well as the proportion of plasma 6-CB associated with it. Corresponding decreases occurred in the proportion of 6-CB associated with low-density lipoprotein and the bottom fraction. During late pregnancy, very low-density lipoprotein became the primary carrier of 6-CB in vivo. The pattern of 6-CB distribution among lipoproteins on day 10 of lactation resembled that on day 9 of pregnancy. Suckling pups exhibited the highest proportion of plasma 6-CB in low- and high-density lipoprotein. The shifts in the distribution of 6-CB during pregnancy and lactation were related to changes in the lipid constituents of plasma and the individual lipoproteins.